The present invention relates in general to sewing machines and in particular to a new and useful controlling mechanism for the step motor of a sewing machine which utilizes stored digital information to produce different stitch patterns.
Electronically controlled sewing machines preferably have step motor drives to control the alteration of the lateral swing-out motion of the needle bar and the feeding motion of the cloth feeder because such drives are excellently suited for the conversion of the digitally stored stitch information. The transmission ratio between the step size of the step motor and the respectively driven element must be selected so that, at a fine enough gradation of the adjusting motion, the adjustment of the driven element within the maximum adjustment range can be made fast enough within the time available. However, under certain conditions the existing gradation from step to step is insufficient. A further division is then necessary.
In one known sewing machine (U.S. Pat. No. 4,191,120), the step setting of the step motor to alter the transport motion of the sewing machine can be corrected manually. This is done by energizing the two phase windings of the step motor differently by means of two potentiometers. Due to this measure, the adjustment of the cloth feed control element can be divided further within the minimum feed range. Due to the better fine adjustment of the control element, better sewing results can be obtained, especially when feeding steps near the zero transport range are made for both forward and backward sewing. The differences resulting in this range between forward and backward feeding, depend upon the type of material to be sewn and upon the operating mode of the cloth feeder so that adjustability must be provided if the quality of the sewing work to be performed is not to suffer. This difference also depends on the exact factory-set step setting of the step motion of the zero transport position of the control element.
Such a correction is especially necessary when sewing patterns are involved which contain a multiplicity of stitches to be made in the one as well as in the other transport direction. In such cases, every feeding difference between the two transport directions, not recognizable in individual stitches, shows as cumulative error which can make the sewing result useless.
The above mentioned known sewing machine solves the problem only very imperfectly because the fine adjustment is restricted only to the minimum feed range of the sewing machine. When set to longer stitch lengths, a correction for the exact execution of forward and backward stitches of the same size is not possible.